PM2.5 60% higher at 100m: Drone study exposes gaps in Delhi’s air monitoring

New Delhi: A new study has found that air pollution above ground level in Delhi can be significantly higher than what conventional surface monitoring stations capture, pointing to critical gaps in how the city measures air quality. Using drone-mounted low-cost sensors, researchers recorded substantially elevated PM2.5 levels at heights equivalent to high-rise buildings, raising concerns for people living or working above the ground floor.
The study, conducted at IIT Delhi over five days in March 2021 during post-winter haze conditions, measured the vertical distribution of fine particulate matter at 20-metre intervals from the surface up to 100 metres. At 100 metres, PM2.5 concentrations were around 160 micrograms per cubic metre—about 60% higher than surface readings on the same days. The findings suggest that residents in 28–30-storey buildings may face higher exposure than indicated by ground-based monitors. Published on Feb 2 in the Nature Portfolio journal npj Clean Air, the study titled "Drone measurements reveal high near surface urban haze" attributes these elevated concentrations to stable, humid early-morning conditions. Researchers observed that the haze occurred mostly in the morning, before the boundary layer expanded with sunlight, allowing pollutants to disperse. During hazy spells, the atmospheric boundary layer—the lowest portion of the atmosphere where pollutants mix—remained unusually shallow. High humidity, low wind speeds and suppressed dispersion created conditions in which pollutants accumulated and secondary particles formed. The study noted that PM2.5 levels increased sharply near the top of this shallow layer, leading to higher concentrations at around 100 metres. "Vertical profiles of PM1 and PM2.5 under humid conditions (relative humidity above 70%) showed that haze formation is likely driven by hygroscopic inorganic aerosols," the researchers said. They added that secondary particle formation, moisture-driven growth, high chloride and nitrate content and weak winds—below 2 metres per second—collectively intensified early-morning haze. On March 18, for instance, particulate matter (PM) concentrations exhibited a notable 60% increase with altitude, reaching 160 micrograms per cubic metre at higher elevations compared to 100 micrograms per cubic metre at the surface. On March 20, a sharp inversion layer around 70 metres led to a sudden 30% jump in PM2.5 within a 20-metre span. In contrast, a major rain event before March 23 reduced PM2.5 to 40 micrograms per cubic metre at the surface—a 60% drop compared with March 18.